This invention relates to fuel cells and, in particular, to electrolyte for use in molten carbonate fuel cells.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
Molten carbonate fuel cells (MCFCs) operate by passing a reactant fuel gas through the anode, while oxidizing gas is passed through the cathode. The anode and the cathode of MCFCs are isolated from one another by a porous electrolyte matrix which is saturated with carbonate electrolyte. Typical MCFC designs include carbonate electrolyte stored in the pores of the anode and of the cathode and in gas passages formed in the anode and cathode current collectors. The electrolyte melts during the initial heat up of the fuel cell and redistributes among the pores of the anode, the cathode and the electrolyte matrix due to the capillary forces of the pores. Conventional MCFCs typically use a eutectic carbonate mixture as the carbonate electrolyte, such as a eutectic mixture of 62 mol-% lithium carbonate and 38 mol-% potassium carbonate or a eutectic mixture of 52 mol-% lithium carbonate and 48 mol-% sodium carbonate.
During MCFC operation, the electrolyte in the cells is consumed by corrosive reactions with the cell components, and as a result of evaporation and electrolyte liquid-phase migration. In particular, liquid-phase migration of the electrolyte occurs due to a voltage gradient within the fuel cell stack, which results in migration of lithium and potassium ions along the length of the stack toward a negative end of the stack and of carbonate ions toward a positive end of the stack. Because lithium and potassium ions in the eutectic carbonate electrolyte move at different rates along the stack length, significant variations in lithium to potassium molar ratios occur within the stack. Such variations in the Li/K ratios affect the stability, conductivity and the melting point of the electrolyte within the cells thereby impacting the performance and the lifetime of the MCFC stack.
In order to avoid large variations in the Li/K molar ratios in the stack, an off-eutectic lithium-rich electrolyte mixture has been employed in MCFCs. U.S. Pat. No. 4,591,538 discloses a lithium-rich electrolyte composition consisting of 70-73 molar % of lithium carbonate and 27-30 molar % of potassium carbonate. The use of electrolyte with a greater lithium content, as disclosed in the '538 patent, improves the uniformity of the Li/K ratio in the stack. However, the electrolyte in the '538 patent has a much higher melting point (575° Celsius) as compared with the melting point of the eutectic electrolyte (485-490° Celsius), requiring higher temperatures during the electrode manufacturing process in order to fill the electrodes with electrolyte. The higher temperatures during electrode manufacturing increase shrinkage and cracking of the electrodes, and as a result reduce electrode production yield and significantly increase the manufacturing costs.
It is therefore an object of the present invention to provide an electrolyte in the MCFC electrodes having increased lithium without requiring higher temperatures during the manufacturing process.
It is also an object of the present invention to provide a method of filling the cathode electrodes with electrolyte without affecting the cathode structure.